The present invention relates to a method of detecting the end point of plasma processing and an apparatus for the same.
A plasma processing apparatus, particularly, an etching apparatus is conventionally extensively used in semiconductor fabrication steps or manufacturing steps of substrates for liquid crystal displays. For example, an etching apparatus of this sort includes upper and lower electrodes arranged parallel to each other. The etching apparatus generates a plasma from an etching gas by discharge between the upper and lower electrodes and uses the resultant active species to etch an object to be processed, i.e., a film such as an oxide film formed on a semiconductor wafer. In this etching processing, it is desirable to perform the processing in accordance with a predetermined pattern by monitoring the progress of the etching and accurately detecting process the end point of the etching.
Conventionally, methods of instrumental analysis such as mass analysis and spectrochemical analysis are used as methods of detecting the end point of etching process. Of these methods, spectrochemical analysis which is relatively simple and has high sensitivity is widely used. When the method of spectrochemical analysis is used in detecting the end point of etching process, a practical approach is to select a predetermined type of active species from active species such as radicals or ions of an etching gas and its decomposition or reaction products and measure a change in the emission intensity of the selected active species with time. The active species to be selected changes in accordance the type of etching gas or the material to be etched. For example, when a silicon oxide film is to be etched by using a fluorocarbon-based etching gas such as CF.sub.4, CO* which is a reaction product of CF.sub.4 and abruptly decreases its emission intensity at the end point is used. One known method is to measure (by using only one wavelength) only the emission intensity (by using a wavelength of 219 nm or 483.5 nm) of CO* with respect to time, and compare changes in the emission intensity and, e.g., the first and second derivatives of the intensity with respect to time, thereby determining the end point of etching process. Another known method as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 63-81929 is to measure the emission intensity of CO* with respect to time and the emission intensity of two-wavelength reference light (in Jpn. Pat. Appln. KOKAI Publication No. 63-81929, an atom such as helium whose emission spectral intensity has wavelengths of 706.5 nm and 667.8 nm), and compare changes in the emission intensity ratio or, e.g., the first and second derivatives of the ratio with respect to time, thereby determining the end point of etching. Another known example of the method using two wavelengths is, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 5-29276 (corresponding to U.S. Pat. No. 5,322,590), to measure the emission intensity of CO* with respect to time and the emission intensity of CF*, instead of the reference light described above, as a decomposition product of an etching gas whose emission intensity abruptly rises at the end point, and compare changes in the ratio of the emission intensities or, e.g., the first and second derivatives of the ratio with respect to time, thereby determining the end point of etching process.
The conventional end point detection method using a single wavelength cannot accurately detect the end point of plasma processing because the end point becomes indistinct due to emission intensity variations caused by, e.g., fluctuation of a plasma. The conventional end point detection methods using two wavelengths do not take account of the fact that a change in the emission intensity of CO* as an active species of a reaction product and a change in the emission intensity of reference light or CF* as a decomposition product of an etching gas, which abruptly raises its emission intensity at the end point, with respect to the end-point time are different due to, e.g., fluctuation of a plasma, a temperature change in a chamber, an electrode, or a wafer, or a deposition sticking to the wall of the chamber. That is, the methods simply obtain the ratio of the two emission intensities and detect the end point by using the ratio. Accordingly, end point detection is difficult to accurately perform.
In U.S. Pat. No. 5,565,114, the present inventors have disclosed an idea by which when the end point of etching process is to be detected by using the emission intensity ratio of two wavelengths, the ratio of the two emission intensities is obtained after changes in the emission intensities with time are matched (i.e., after the slopes of curves representing changes in the two emission intensities with time are matched). When the ratio of the two emission intensities is monitored after the slopes of change curves of the two emission intensities are matched in advance as disclosed in this publication (U.S. Pat. No. 5,565,114), the end point can be detected more accurately than when the ratio of the two emission intensities is simply monitored. Unfortunately, the method of matching the slopes of the two emission intensities disclosed in this U.S. patent calculates average values of change curves of the emission intensities in a designated interval before the end point, and calculates a total sum of absolute values of the differences between the emission intensities and the average values in the designated interval for the two change curves. Since the method uses this total sum (i.e., calculates an area), the method is weak against noise.